Method of and apparatus for reducing contact resistance



July 23, 1935. w. R. WALKEF METHOD OF AND APPARATUS FOR'REDUCING CONTACT RESISTANCE Filed May 17', 1954 INVENTOR Patented July 23, 1935 UNITED STATES METHOD OF AND APPARATUS FOR REDUC ING CONTACT RESISTANCE Warren R. Walker, Short Hills, N. J., assignor to General Electric Vapor Lamp Company, Hoboken, N. J., a corporation of New Jersey Application May 1'7, 1934, Serial No. 726,155 7 Claims. (Cl. 200-152) The present invention relates to fluid flow switches generally, and more particularly to the method of production thereof.

A particular object of the invention is to produce a fluid flow switch having a low internal resistance. A further object of my invention is to provide a novel method ofremoving a high resistance film from the inleads of a fluid flow switch. Another object of my invention is to 10 provide a novel apparatus for the treatment of fluid flow switches and the like to reduce the resistance thereof. Still other objects and advantages of my invention will appear from the following detailed specification, or from an inspection of the accompanying drawing.

The invention consists in the novel series of steps and in the novel combination of parts which is hereinafter set forth and claimed.

In the manufacture of fluid flow switches, such as the well known mercury switches of commerce, it is found that upon completion these switches usually have a resistance of the order of ten fold the value which the resistance of the inleads and of the fluid path would indicate. I

have found that this abnormally high resistance is due to a film, which frequentlyconsists at least in part of oxide, on the inleads which prevents direct contact between the inleads and the fluid. It is virtually impossible, moreover, to

prevent the formation of this relatively high resistance film during the fusing operations incident to the manufacture of hermetically sealed vitreous switches, particularly in the more compact type of switch, hence some means of re-,

35 moving this film from at least some part of the inlead is necessary if the finished switch is not to have an excessiveuesistance. Several methods of producing this result have been heretofore proposed which are entirely satisfactory in many cases, but unfortunately there are types of fluid flow switches from whose inleads this film can not be removed by any of these prior methods. I

have now discovered that this film can be removed from the inleads of any type of fluid flow switch in a novel and very simple manner using a novel' apparatus of my invention. According to this novel method the switch is plawd in the closed circuit position and a large current,

50 many times that for which the switch is designed, is passed therethrough for a fraction of a second. I have discovered that this large current causes the destruction of the undesired film, at least over a considerable area, and thus results {5 in the reduction of the resistance of the switch to a value which is substantially that which is to to be expected in the absence of such a film.

For the purpose of illustrating my invention I have shown in the accompanying drawing a schematic diagram of the electrical circuit em- 5 ployed to produce this novel result.

As shown in this drawing the primary I of a suitable step-down transformer is connected to a suitable source of alternating current through a resistance 2 and a switch 3 which is designed 10 to make only a. momentary contact. While this switch 3 has been illustrated, for convenience, as a simple mercury switch of the well known runacross type, it is to be understood that various other types of switches may be used, including 5 those in which the time interval is entirely within the operator's control. I prefer, however to use a switch of the type here shown, or such as disclosed in my Patent 1,930,435, granted October 10, 1933, in which the current duration is positive- 2 ly limited to the desired fraction of a second. The low voltage secondary 4 of said transformer is connected toa pair of spring clips 5 and 6 which are adapted to engage the inleads l and 8, respectively, of a mercury switch 9. -This mer- 25 cury switch, which is of the type disclosed in Patent 1,984,419, granted December 18, 1934 to Frank Moos, has a tubular body within which there is positioned a hollow cylinder Ill of porcelain or the like. The inlead 8 passes through the upper end of said cylinder l0 and is suitably sealed therein to close the upper end of said cylinder. The other inlead I extends alongside of said cylinder In to a point near" the lower end thereof and then encircles said cylinder. A quantity of mercury l l which is suflicient in quantity to make contact with both of these inleads when the switch is in the position shown is sealed within said envelope. An arc suppressing atmosphere, such as hydrogen, at substantially atmospheric pressure is likewise preferably sealed within said envelope.

With a switch of the foregoing construction a pool of mercury is always retained in contact with each of the inleads I and 8 regardless of the position of the switch, in order to insure that the circuit will always be closed by the merging of two mercury bodies. The presence of these mercury pcols, however, while extremely desirable in the operation of the switch, has made it impossibie to draw an arc to either inlead, according to a method heretofore in use with other types of mercury switches, to remove the film formed thereon during manufacture. It is, however, essential that this film should be removed if the switches are to have a low and uniform internal resistance. I have now discovered that this result can be attained by the simple expedient of momentarily passing a very large current therethrough, this current ordinarily being many times that which could be continuously carried by the switch. Thus with the switch illustrated, which has a rated continuous current carrying capacity of 15 amperes, I find that if a current of approximately amperes is passed therethrough for approximately one sixth of a second the increased resistance due to the film is eliminated. Just what happens to this film under these circumstances is not known, but it is an observed fact that it no longer interferes with current fiow through the switch.

This novel result is conveniently obtained in practice by placing the inleads 1 and 8 of the switch 9 in contact with the clips 5 and 6, respectively, with said switch 9 in the closed circuit position. The switch 3 is then momentarily closed by the operator by moving it from one rest position to the other, whereupon a heavy current flows through the switch 9. The potential employed is not particularly important, although in general this potential should not be suflicient to produce sustained arcing at the films within the switch 9. Likewise it is desirable that it should be high enough to permit the use of some external resistance so that the current flow will not vary too widely as the film on the inleads I and 8 is removed. Thus the secondary 4 in the case illustrated has an open circuit potential of l4 volts, while the resistance 2, of the order of .1 ohm, is so chosen as to limit the iinal current, after elimination of the film on the inleads I and 8, to approximately 140 amperes. The use of alternating current, such as described hereinbefore, is a matter of convenience and not of necessity since the same results can be attained with direct current. The control of such a large current in a direct current circuit is somewhat inconvenient, however, and in addition there is the need for providing a suitable low voltage source, hence the alternating current system described is much more practical. It is likewise more practical to time the duration of the current with the switch 3 rather than by movement of the switch 9, both because of the smaller current to be controlled, and because the switch 3 is better suited for timing. The duration of the current flow is not critical, although it is essential that it should not persist long enough to damage the switch 9 through overheating the seals. Thus in practice I usually limit the current fiow to approximately one sixth of a second. With a switch of the type illustrated this current flow has been found to be entirely effective, the resistance of the switch being decreased from .05 ohm, or more, before treatment to approximately .004 ohm after treatment. Such a reduction in the switch resistance is extremely important, of course, but even more important is the fact that my new process reduces the resistance of all the switches of any particular type to the same value, whereas so long as the film persists on the inleads the resistance of the different switches may vary considerably. Thus my novel process also affords a means of producing a uniform product.

While I have illustrated my novel process as applied to a type of switch which could not be treated with any of the film removing methods of the prior art, it is to be understood that its rated continuous use is not limited thereto, but that it is useful in removing the film from the inleads of any type of mercury switch or other similar device. Furthermore the ease and certainty with which the desired result is obtained with my novel proccess makes it appear probable that this new process will henceforth be used almost exclusively to remove the film from the inleads of all devices of these types. It is furthermore to be understood that various changes, omissions and substitutions, within the scope of the appended claims, may likewise be made in the steps oiLthe process without departing from the spirit of my invention.

I claim as my invention:-

1. The method of producing a low contact resistance between a liquid metal and a solid body having a film of resistance increasing material thereon which comprises passing a current through the junction between said liquid metal and said solid body of sufiicient intensity to remove said resistance increasing material from the surface of said solid body.

2. The method of removing a high resistance film from the inleads of a mercury switch or the like to reduce the internal resistance of said switch which comprises passing through said switch.a current which materially exceeds the current carrying capacity thereof for a period long enough to remove said high resistance film from appreciable areas on the inleads of said switch.

3. The method of removing a high resistance film from the inleads of a mercury switch orthe like to reduce the internal resistance of said switch which comprises placing said switch in a closed circuit position and then momentarily passing through said switch a. current which materially exceeds the rated continuous current carrying capacity thereof.

4. The method of reducing the internal resistance of a mercury switch or the like which comprises placing said switch in a closed circuit position and then passing a current of the order of 140 amperes therethrough. for a fraction of a second.

5. An apparatus for reducing the contact resistance between a fluid and a solid body within a sealed envelope which comprises in combination, means to pass a current of suflicient intensity to remove resistance increasing material from the surface of said solid body through the junction between said fluid and said solid body, and means to limit the duration of said current flow.

6. An apparatus for reducing the internal resistance of a fluid flow switch comprising'means to support said switch in a closed circuit position, means to pass through said switch a current which materially exceeds the continuous current carrying capacity thereof, and means to limit the duration of said current.

7. The method of removing a high resistance film from the inleads of a mercury switch or the like to reduce the internal resistance of said switch which comprises connecting said switch to a source of energy which is incapable of maintaining a sustained arc in said switch and momentarily'passing through said switch a current of sufilcient intensity to remove said film from appreciable areas on the inleads of said switch.

WARREN R. WALKER. 

